Selective catalytic reduction (SCR) is the best available technology for emission control of the oxides of nitrogen (NO.sub.x). NO.sub.x is generally recognized as the most significant precursor gas that leads, in combination with volatile organic compounds, to the formation of ozone in the troposphere. NO.sub.x is also believed to be responsible for the nitric acid component of acid raid, particularly in the Western United States. Considerable legislation on a regional and national basis limits NO.sub.x emissions. A need therefore exists for SCR systems that can be mass produced on an economical basis and provide high conversion efficiency over extended periods of time.
Although the devices of the present invention may be used in conjunction with any hydrocarbon fueled system which on burning yields an exhaust gas stream which contains significant amounts of NO.sub.x, the primary system with which these devices are used is a gas turbine driven generator system for generating electrical power.
Catalytic converters now being used for NO.sub.x control in Japan, West Germany and the United States are comprised of vanadia and the oxides of tungsten or molybdenum on a washcoat of titania. These are known as "vanadia SCR systems". The catalytic material is displayed on molded or extruded ceramic honeycomb, formed metal plates or a stainless-steel-foil honeycomb structure. Vanadia SCR systems typically operate at optimum conversion efficiency of 700.degree. F. (370.degree. C.), or less efficiently in an outer range from 570.degree. F. to 750.degree. F. (300.degree. C.-400.degree. C.).
An SCR system is needed that can operate in a temperature zone lower than the 700.degree. F. (370.degree. C.). This need arises because the vanadia SCR has to be located in the mid-section in most boiler trains, which is the only practical location in the boiler train where the proper temperature zone exists. However, this often results in the boiler manufacturer having to design the boiler with provisions for splitting the boiler to allow for insertion of the vanadia SCR, often adding expense beyond that associated with the vanadia SCR and its associated ammonia injection system.
Furthermore, an even greater need is arising because many major boiler trains now in the field will have to be retrofitted with SCR units when pending deNO.sub.x legislation is passed, such as California Rule 1134. Otherwise, retrofit installation of vanadia SCR systems will require the cutting apart of boiler trains, relocation of the displaced section on new foundations and associated repiping--generally considered a major project with costs at a multiple of the vanadia SCR system itself.
The present invention satisfies these needs through location of an SCR system downstream of the heat recovery steam generator (HRSG), at a location where the boiler does not have to be redesigned in two sections, or cut apart in the case of retrofit with an SCR system.
The system of the present invention includes a low temperature SCR catalyst that reaches maximum conversion efficiency for NO.sub.x at approximately 400.degree. F. (200.degree. C.), is located downstream of the HRSG and has its ammonia injection system at a location as far upstream (toward the gas turbine or other combustion device or source of NO.sub.x) as possible, in the interest of causing the maximum diffusion of NH.sub.3 in the exhaust stream, such that when the exhaust stream flows through the low temperature SCR system the gas will be a homogeneous mixture to assure maximum NO.sub.x conversion and the minimum possible amount of NH.sub.3 for complete NO.sub.x conversion, since NH.sub.3 is an irritant with an unpleasant odor, even at very low concentrations.
The low temperature SCR (LT-SCR) is located downstream of the HRSG. The LT-SCR can be located upstream or downstream of a fuel economizer or other heat exchanger such as a regenerator for preheating combustion air, but such that the temperature of the LT-SCR is in the range of 300.degree. F. to 550.degree. F. (150.degree. C. to 290.degree. C.).
The ammonia injection grid should ideally be located as near as possible to the turbine outlet, which in combined cycle and simple cycle gas turbine systems is generally close to 1000.degree. F. (540.degree. C.). In this way the maximum NH.sub.3 mixing will take place with the exhaust gas in the zone of the most turbulent flow before impinging on the LT-SCR. However, if a carbon monoxide converter system is present, the ammonia injection must take place downstream of the CO converter, but as far upstream of the LT-SCR as possible, to assure maximum mixing.
Reference may be had to the patents to Inui et al, 4,106,286 dated Aug. 15, 1978 and 4,466,241 dated Aug. 21, 1984 for disclosure of closely related devices which are adapted to have SCR's which operate at higher temperatures because of the location of the SCR between portions of the evaporator, (4,466,241) or between the evaporator and the fuel economizer (4,106,286). The location is such that the SCR is in a temperature zone where the temperature is between 570.degree. F.-750.degree. F. (300.degree. C.-400.degree. C.). It so happens that a vanadium SCR system is the catalyst of choice, and its temperature of most suitable operation is about 700.degree. F. for the reduction of NO.sub.x. Hence the location of the SCR is determined to be in that zone where the temperature is most favorable to the operation of the catalyst. The disclosures of U.S. Pat. Nos. 4,106,286 and 4,466,241 are incorporated herein by reference. With exception of the location of the SCR unit, the elements of the waste heat recovery system of the present invention are essentially the same as those utilized in the prior art and their structure is well known.